In the text hereinafter, specific reference is made to the reduction of zirconium tetrachloride, ZrCl4, by aerosol sodium, Na, to form elemental Zr and byproduct NaCl. Many other metals in addition to zirconium may be produced according to this invention. Metal halide compounds to which this invention applies include HfCl.sub.4, HfF.sub.4, HfI.sub.4, NbCl.sub.5, SiI.sub.4, SiCl.sub.4, SiF.sub.4, TaCl.sub.5, TiBr.sub.4, TiCl.sub.4, TiF.sub.4, TiI.sub.4, UF.sub.6, VCl.sub.3, ZrBr.sub.4, ZrCl.sub.4, ZrF.sub.4, and ZrI.sub.4.
Metallic reducing agents which are transferred in the practice of this invention applies include Al, Ca, Mg, and Na.
Presently, the metals listed are produced by several processes, each with its own set of shortcomings.
Zr, Ti, and Hf are produced by the Kroll process, which is described in U.S. Pat. No. 2,205,854. This process is time consuming and expensive. From the input of ZrCl.sub.4 and Mg to the output of elemental Zr metal may take up to 10 days.
Additionally, the product of the Kroll process is a spongy form of the product metal, which readily absorbs undesirable gases such as oxygen and nitrogen from the atmosphere. The "sponge" must be crushed down to a small size (3/4" particles), visually inspected for "burnt" particles high in oxygen and nitrogen, then blended and compacted before finally it is melted to produce homogeneous metal. Incremental improvements to the Kroll process are described in U.S. Pat. Nos. 2,763,760; 2,860,966; 2,922,710; 3,114,611; 3,318,688; 3,966,460; 4,242,136; 4,441,925; 4,897,116; and 5,098,471 .
Nb, V, and Ta are frequently produced by the "thermite" process, in which the oxide of the product metal is exothermically reacted with a metallic reducing agent whose oxide is more thermodynamically stable than that of the product metal. Typically, powdered aluminum (for example) is blended with an oxide such as Nb.sub.2 0.sub.5 (for example), and the mixture is ignited to produce Nb and Al.sub.2 0.sub.3.
The product metal in most thermite reactions is a "derby" of homogenous metal, but the derbys typically must be melted several times under very high vacuum conditions to obtain acceptably pure metal. Additionally, thermite processes are limited in the maximum amount of metal which can be produced in a single batch for thermodynamic reasons known to those skilled in the art of extractive metallurgy; maximum batch sizes are only several hundred pounds. Typical thermite processes are described in U.S. Pat. Nos. 2,789,896; 2,801,915; 3,014,797; and 3,764,535.
One method by which very pure silicon is produced is by the thermal decomposition of a silicon halide. Such decomposition methods are taught in U.S. Pat. Nos. 2,889,221; 2,895,852; 2,916,359; 2,999,735; 3,049,440; and 3,523,816.
The other metals to which this invention applies are produced by methods similar to those described. In general, all of the metals are produced in batch, as opposed to continuous processes, and each requires substantial processing after the final chemical reduction reaction produces elemental metal, in order to obtain an ingot of homogeneous metal of controllable purity.
The limitations present in the present production methods for Zr, Ti, Hf, Si, and the other metals listed are well known to those skilled in the art. The literature in this area of extractive metallurgy covers numerous approaches; some represent continuous variations on the present production methods, while others are progressively more theoretical.
One class of approaches for improved production methods for many of these metals are so-called "flame reduction" processes. In these processes, the metal halide is vaporized, as is the metallic reducing agent (typically Na or Mg). The two gases are brought together in some type of burner arrangement, which to some extent may resemble a modern blowtorch, stovetop, or similar burner. The chemical reaction is extremely rapid, exothermic, and luminescent; hence the designation. A typical overall equation for such reactions is: EQU ZrCl.sub.4 (g)+4Na(g).fwdarw.Zr (1,s)+4NaCl(g).
This type of approach is attractive for many reasons. If such a reactor could be made to operate continuously, it could be much smaller than presently used equipment, with correspondingly lower initial and operating costs. By virtue of operating continuously, such a reactor could make far purer metal than present equipment, since it would not be periodically opened to the atmosphere. Also since both reactants could be metered to the reactor in controlled amounts, the metallic reducing agent would be more efficiently used. Most present methods add up to 25% excess metallic reducing agent to drive the reaction toward completion. The product metal would be more uniform, since there would be far fewer start up/shut down periods, when the dominant reaction mechanisms may vary, causing differences in the product. Finally, a continuous process that produced homogeneous ingot metal would eliminate a large number of subsequent processing steps presently required after the reduction reaction which initially yields unconsolidated elemental metal.
Typical flame reduction processes are described in U.S. Pat. Nos. 2,760,858; 2,826,491; 2,828,199; and 2,828,201 by Findlay; in 2,762,093; 2,773,759; and 2,782,118 by Hood; in 2,766,111 by Singleton; in 2,816,828 by Benedict; in 2,870,007 by Boettcher; in 3,085,071-3 by Griffiths; in 4,830,665 by Winand; in 5,021,221 by Gould; and in 5,032,176 by Kametani. However, none of these processes has achieved industrial acceptance.
There are three major reasons for the failure of these prior art flame reduction processes to compete with the existing production methods for the metals in question. The first is that, in order to vaporize the commonly available metallic reducing agents Na and Mg, very high temperatures must be employed, e.g., up to 1400.degree. C. These temperatures are difficult to achieve by current industrial methods, given the additional requirements for atmospheric purity, flow control, and other complications.
Another problem is that since the metallic reducing agent is vaporous, even higher temperatures are generated during the reaction (up to 2000.degree. C.), and frequently the torch apparatus is consumed or melted to an undesirable degree. The third problem is that metal produced in flame reduction reactions is produced in the form of very small particles, typically smaller than 1 micron. Such small particles are extremely reactive, and unless extraordinary, and currently expensive procedures are employed in subsequent processing to ingot form, the particles become unacceptably contaminated with oxygen and nitrogen, rendering the product metal unsuitable for use without further processing.
All three problems, to date, have delayed industrial acceptance of a flame reduction process for the metal halides identified herein. No prior process successfully addresses the destructive effects of either the high temperatures required to produce a vaporous flow of the metallic reducing agent, or the even higher temperatures generated by the intense exothermicity of the reaction.
Those prior art flame reduction processes that have effectively collected the sub-micron product metal particles, have done so by attempting to capture the submicron product metal particles in a liquid pool of the product metal. Two variations to the solution of this last problem are evident in the prior art.
In the first variation, it is claimed that the intense exothermicity of the reaction is sufficient to maintain a pool of liquid product metal in a bottomless crucible or hearth. As the sub-micron particles of product metal accumulate in the pool, a solidified ingot is withdrawn from the bottom of the crucible, thereby maintaining a constant liquid
pool location. Although this is the most attractive solution, it neglects the great amount of heat which is typically removed from the crucible by water cooling.
Many of the metals listed herein are extremely reactive when molten, vigorously attacking or dissolving the available structural solids. These metals may only be contained within a frozen layer, or "skull", of the same metal. Typically, a crucible for the containment of these liquid metals consists of a cylindrical water cooled copper sleeve, which may be 2-30in diameter and 1/4 to 11/2" thick. Molten metal which touches the water cooled sleeve freezes instantly, thus providing the "skull" effect of a container of the same metal.
To achieve the "skull" effect, a great deal of heat must be removed from the liquid pool of the product metal to provide for a frozen layer of the product metal in contact with the water cooled crucible. In practice, as those skilled in the art of extractive metallurgy are aware, this amount to maintain the pool and the skull is typically several times over the amount of heat generated by flame reduction. In order to produce a homogeneous ingot of the product metal, the water cooling necessary for practical reasons will remove too much heat, and a liquid pool cannot be sustained.
The second prior art variation for maintaining a pool of liquid product metal which will serve to collect sub-micron metal produced in a flame reduction reaction into the form of a homogeneous ingot has been to supply extra heat to the pool of liquid product metal by arc melting a rod or bar of the product metal into the pool while the flame reduction reaction occurs. This is described by Findlay in U.S. Pat. No. 2,828,199. Although this method is indeed capable of supplying the necessary extra heat to maintain a pool of liquid product metal in a water cooled crucible, it has been found that the arc dynamics interfere with the flame reduction reaction to an extent that renders this method unacceptable.
As known to those skilled in the art of arc melting, an arc discharge generates extremely high temperatures, up to and beyond 10,000.degree. C. The intense heating causes vigorous convective circulation currents, which greatly interfere with the flame reduction reaction. The result is that the sub-micron product metal cannot be efficiently deposited onto the surface of the liquid pool. Rather, a large percentage of the metal is transported away from the pool by arc-induced gas flows. Thus, this approach does not solve the problem of how to maintain a pool of liquid product metal in a water cooled crucible, where said pool is necessary to efficiently capture sub-micron product metal produced in a flame reduction reaction to produce a homogeneous ingot of product metal.
To summarize the prior art in flame reduction processes, although the chemical reactions occur favorably, there has not yet been a reactor design which mitigates the problems of the high temperatures employed and generated, nor which successfully captures the sub-micron particles produced to form a homogenous ingot of product metal.